Various techniques for forming high performance metal alloys are known. These include powder metallurgy and ingot metallurgy. Of these, powder metallurgy is of particular interest because of its unique ability to form alloys having a microstructure unachievable using more conventional techniques, such as casting.
Generally, high performance alloys may be formed by combining a matrix metal, such as aluminum, with a refractory material which forms a discontinuous phase in the matrix. Examples of refractory materials include alumina, silicon carbide, boron carbide, aluminum nitride and silicon hexaboride. The alloys formed have increased strength and modulus of elasticity compared to monolithic aluminum alloys.
However, the extrusion of such alloys is difficult, due in part to high flow stress generated during extrusion. The flow stress is a function of the high temperature strength of the alloy and reinforcement loading. As the percentage of refractory material in the matrix is increased, flow stress increases along with strength. If severe enough, the flow stress hinders commercial scale manufacturability of the alloy.
For example, discontinuous reinforced aluminum metal matrix composites are difficult to work and quickly wear out conventional steel die toolings. Hypereutectic and superhypereutectic alloys formed of these matrices generally cannot be extruded economically due to poor productivity, excessive scrap (poor recovery), extrusion die failure and excessive die wear. However, hypereutectic and superhypereutectic aluminum silicon alloys (i.e., alloys having greater than 25% and 35% silicon, respectively) would have great utility in many applications.
Prior art attempts to decrease the flow stress of such alloys have been generally unsuccessful. The approaches usually taken are to raise the billet temperature above the solidus of the alloy and/or to increase the strength of the die insert by the use of ceramics. However, when the billet temperature exceeds the solidus, the morphology of the alloy is altered. Billets formed by powder metallurgy have a substantially uniform and homogeneous microstructure, and the particulates in the billet coalesce at temperatures above the solidus, leading to grain enlargement, which in turn decreases the strength of the alloy.
The use of ceramic die inserts also has adverse consequences, such as thermal shock. This occurs when the ceramic material is rapidly heated and cooled and results in stress cracks in the ceramic which ultimately leads to catastrophic failure. To avoid thermal shock, the ceramic die inserts must be heated and cooled slowly, which greatly increases the lead time for any production run.
Hence, there remains a need in the art for a method and apparatus for extruding high performance metal alloys while leaving the physical properties of the alloys intact. There is also a need in the art for such a method and apparatus which does not require extended periods of lead time. These needs are met by the present invention.